1. Field of the Invention
The present disclosure generally relates to an image forming apparatus using electrophotography, and more particularly to an image forming apparatus for forming images on recording media having different glossiness.
2. Description of the Background Art
In general, image forming apparatuses are configured to output images, such as a toner image, on a recording medium by changing glossiness of images.
Recently, market demand has been growing for producing images having a variety of glossiness by using image forming apparatuses employing electrophotography. More specifically, although users have long been using image forming apparatuses for producing images on plain paper having lower glossiness, widely used in business fields, more are now demanding higher quality images for image forming apparatuses, wherein such higher quality images may be images formed on a recording medium having higher glossiness, such as art paper, coated paper, or fine coated paper. In this disclosure, a recording medium having lower glossiness has greater microscopic asperities on the surface, and a recording medium having higher glossiness has smaller microscopic asperities on the surface.
However, conventional techniques may not be suitable for producing images having an optimal glossiness and uniform glossiness on various recording media having different glossiness because graininess of images may deteriorate in the conventional techniques. In other words, it may be difficult to form images having optimal glossiness and uniform glossiness on recording media in a stable manner by the conventional techniques.
In this disclosure, “uniform glossiness” or “evenness of glossiness” means the level of evenness of glossiness of a recording medium having a toner image thereon. Further, “graininess” means the level of graininess of toner image formed on a recording medium. Both “evenness of glossiness” and “graininess” are important factors for evaluating image quality.
The following factors may cause lower “evenness of glossiness.” (1) Difference of image concentration resulting in difference of image glossiness on a recording medium, by which sense of discomfort is perceived by eye observation (visual observation), in which the difference of image concentration is caused by difference of toner adhesion amount on the recording medium. (2) Difference of glossiness between a recording medium and a glossiness of image-formed area causes sense of discomfort perceived by eye observation (visual observation). Accordingly, to enhance evenness of glossiness of image, it is required to set a similar glossiness level for an entire image-formed area including higher image concentration area and lower image concentration area, and to set a similar glossiness level for a recording medium and image-formed area (toner adhesion area).
As for a lower glossiness recording medium, such as plain paper or the like, a conventional fixing unit having no function for changing glossiness level or for producing higher glossiness can be used to produce images having allowable level of image quality. However, such conventional fixing unit may not be suitable for producing a higher quality image on a recording medium having higher glossiness, such as coated paper. More specifically, such conventional fixing cannot produce a higher quality image as follows.
A recording medium having smaller surface irregularity has a higher glossiness. For example, such recording medium has a 60-degree specular glossiness value of 10% to 80%. When a toner image is formed on such recording medium, it may increase surface irregularity, by which the glossiness level of whole image decreases. As a result, a difference is observed in a level or degree of glossiness between the recording medium and a toner image area. If the difference or contrast between glossiness becomes too great, an observer is apt to feel that the image quality is poor.
Further, another drawback may occur when an image is output on a recording medium having higher glossiness (or smaller surface irregularity).
In an electrophotography process, a toner image is formed using an area coverage modulation method to reproduce the image having suitable image concentration. The area coverage modulation is used to reproduce image gradation of an image by changing an area ratio between a toner-adhered area and a toner-not-adhered area. When an image area having middle (or intermediate) concentration area is formed, such image area may include the toner-adhered area and the toner-not-adhered area, wherein the toner-not-adhered area is a surface of recording medium not adhered with toner (i.e., exposed surface of recording medium). Accordingly, toner particles are adhered in a patch-work manner on the recording medium having a smaller surface irregularity, and the toner-adhered area on the recording medium has a greater surface irregularity.
When an image area ratio is small, a higher glossiness can be attained because a ratio of the toner-adhered area to a recording medium as a whole having higher glossiness is small. However, at intermediate image area ratios, a lower glossiness may be observed on the resultant image because a greater number of surface irregularity portions may exist between toner-adhered area and the exposed surface of the recording medium. Further, at high image area ratios, the glossiness level may become higher or lower depending on melted condition of toner. Therefore, because the glossiness is changed due to an image area ratio, glossiness difference may occur in one output image, and thereby an image having a lower level of evenness of glossiness may be output.
One conventional approach involves use of a technique that outputs images by switching over image glossiness between lower and higher glossiness using a transfer/fixing unit so that images can be output with desired glossiness for all types of images, such as photography image, monochrome image, and so forth, by which a user may not feel sense of discomfort on glossiness of output images.
In such technique, a switching over of glossiness is conducted by (1) changing a contacting time between a second image carrying member and a recording medium or by (2) changing a pressure applied to a toner image carried on a second image carrying member. With such configuration, images having optimal glossiness can be output on various recording media having different glossiness. However, such technique may output image having degraded graininess in some cases. Especially when fixing conditions are changed from a lower glossiness image to a middle glossiness image (or a higher glossiness image), a toner image may be transferred to a contacting member during a fixing process, by which an output image may have a lower graininess.
Another conventional technique outputs images having desired glossiness without changing a temperature or linear velocity of a fixing device of a second fixing unit disposed after a first fixing unit, in which the second fixing unit is disposed at a position where temperature of toner images is still at a glass transition point or higher.
In such technique, the first fixing unit applies heat to a toner image under a pressurized condition to fix the toner image on a recording medium, and then the toner image on the recording medium is re-pressurized by the second fixing unit having no heating device. Accordingly, images having optimal glossiness can be output on various recording media having different glossiness.
However, the second fixing unit only re-pressurizes the toner image because a heating device is not disposed. Accordingly, a temperature condition of toner image fixed by the second fixing unit may not be stabilized, and thereby an image output from the second fixing unit may not have a desired glossiness. Especially when installation environment of an image forming apparatus changes, a temperature condition of toner image fixed by the second fixing unit also changes and thereby an image output from the second fixing unit may not have a desired glossiness.
Further, another conventional technique in which a second fixing unit, disposed after a first fixing unit, fixes toner images on both faces of a recording medium simultaneously so as to efficiently form higher quality toner images on both faces of a recording medium as a photographic image. Although such technique can be used to form a toner image having higher glossiness on a recording medium having higher glossiness (e.g., 60-degree specular glossiness value of 80% or more), such technique may not be useful for producing images having optimal glossiness on plain paper or the like. For example, such technique may not form a toner image having an optimal glossiness on a recording medium of intermediate glossiness (e.g., 60-degree specular glossiness value of 10% to 60%), which has a lower glossiness than a recording medium having higher glossiness.
Finally, another conventional technique has a fixing unit that applies heat to a toner image under a pressurized condition to fix the toner image on a recording medium, after which the toner image on the recording medium is cooled and separated by a cooling and separation unit so as to form a smooth color image having higher glossiness on the recording medium. The cooling and separation unit uses a belt contactable to a toner image so that toner image can be transferred with surface shape of the belt, such as roughness. Accordingly, glossiness of toner image can be controlled by changing a surface property of the belt between mirror-finished surface (or higher glossiness), middle glossiness, and lower glossiness. Specifically, several belts having different surface property (or different surface irregularity) can be prepared, and by replacing belts, glossiness of toner image can be optimized for recording media having various glossiness.
However, such belt replacement configuration may be inconvenient for user operability. Further, if a belt having surface property of middle glossiness to lower glossiness (60-degree specular glossiness value of 60% or less) is used instead of mirror-finished surface for cooling and separation, some toner may adhere on the belt of the fixing unit. If such toner adhered phenomenon occurs, graininess of image on a recording medium may be degraded.
In view of such situation, an image forming apparatus which can produce images having a good level of graininess and uniform glossiness on various recording media having various glossiness is desired.